Mutations in the PKD1 gene account for the majority of patients diagnosed with autosomal dominant polycystic kidney disease (ADPKD). PKD1 encodes for polycystin, a large glycoprotein that contains several extracellular motifs characteristic of molecules involved in cell-cell or cell-matrix interactions. The precise function of polycystin in currently unknown. To delineate the downstream signaling pathways of polycystin, a yeast two-hybrid screen was used to isolate two proteins, P12 and P18, that bind to the C-terminal cytoplasmic domain of polycystin. P12 is homologous to the dystrophin-/spectin gene super-family of actin-binding proteins. Proteins of this gene family appear to function as linkers between integral membrane proteins and the actin-cytoskeleton, and thus promote normal cell structure and differentiation. The requisite interaction of polycystin with P12 may be essential for maintaining cellular polarity and structural integrity of tubular epithelial cells. The second polycystin-interacting protein, P18, belongs to a family of proteins that regulate signaling through G-proteins. This interaction argues for a role for polycystin in G protein-coupled signaling. The proposed experiments aim to relate the normal function of polycystin to its binding and subsequent signaling through P12 and P18 during renal development. To elucidate the role of polycystin, structural studies will involve detailed mapping and mutational analysis to delineate the significant domains of polycystin, P12 and P18; functional studies will entail establishing the temporal and spatial expression patterns of the three proteins during renal development and demonstrating their interaction in vivo.